This invention relates generally to seismic prospecting and, more specifically, to geophone arrays for use in shallow marine, rivet, and land environments.
Originally, seismic acquisition contractors used geophone and hydrophone arrays with the individual geophones and hydrophones being connected by a flexible wire for the purposes of electrical communication and, in the case of hydrophones, position maintenance through drag. A serious impediment exists in fast moving bodies of water such as rivers and tidal marine areas.
Seismic acquistion in the marine environment has typically utilized a number of hydrophone arrays strung together by wire and towed behind a boat. Movement of the boat deploys a drag type seismic cable containing the hydrophones and hydrophone arrays in a straight line. The hydrophone arrays are coupled to the body of water overlying the sediment and rock. Since the reflection coefficient at the seabed or water/earth interface is very large, the reflection energy reaching the hydrophone arrays is significantly attenuated due to this large reflection coefficient. Tidal marine conditions exist in coastal areas where strong horizontal ocean currents can easily destroy seismic equipment. Fast moving bodies of water like rivers also have very strong currents that have made previous attempts to acquire seismic reflection data in these water environments generally unsuccessful.
Navigation and position maintenance of a drag type seismic cable containing hydrophone arrays in tidal marine and river environments is very treacherous. Strong currents easily disrupt loosely connected geophone arrays strung together by wire. Sheltering of marine and land geophones from noise producing water and wind turbulence is limited.
Exemplary of the prior art is U.S. Pat. No. 3,416,629 to Brede, which is directed to an apparatus for stabilizing a drag type seismic cable containing hydrophone arrays through the use of a boat at one end of the tow line and a data collection truck based on the shore. The apparatus of Brede is subject to undesireable attenuation of the hydrophone signal due to the reflection coefficient of the seabed.
U.S. Pat. No. 4,463,451 to Warmack teaches a way to stabilize and maintain the relative position of a single geophone in a water covered area using a recording float and an elaborate tension filter, which is expensive and difficult to deploy.
U.S. Pat. No. 2,738,488 to MacKnight is directed to a single drag type cable employing single geophone attachments, which have questionable coupling and position control in tidal marine areas and fast moving rivers.
U.S. Pat. No. 5,014,813 to Fussell is directed to a waterproof housing for single seismic sensor typically used in marshes and other areas of quiet water. The typical marsh geophone is elongate and is made to be coupled or positioned in the mud. Groups of geophones are connected by wire for the purpose of electrical communication. Planted singly, these geophones arrays can easily become decoupled and are ineffective in tidal marine areas and rivets.
U.S. Pat. No. 4,138,658 to Avedik is directed to a complex pickup, comprising a detachable frame, a hydrophone and two geophones that are used in water depts of 100-200 meters in connection with refraction surveys. This arrangement suffers from poor earth coupling, since the geophones are not individually planted in the earth. Only the frame is directly coupled to the earth through the three feet that are provided on the underside thereof, Therefore, the geophones sense refractions through the frame, rather than directly from the earth.
Seismic acquisition on land has in the past utilized a plurality of geophone arrays strung together by wire for the purpose of electrical communication to a multichannel recording unit. Individual geophone arrays normally consist of a plurality of geophones that are planted in spaced groupings of 12-24 geophones over distances of 55-440 feet by unskilled operators who have little regard for proper orientation of the geophones. Terrain changes are not usually taken into account. When these arrays of geophones are planted on hillsides, plane reflection waves coming from different directions impinge on the group array at different angles, thereby causing misalignment of the response signals.
In an attempt to improve data quality, three-dimensional geophones have been used to measure motion in three orthogonal directions. Each three-dimensional geophone typically comprises three separate unidirectional geophones that are oriented for three-dimensional pickup and housed in a single enclosure, as typified by the geophone described in U.S. Pat. No. 5,010,531 to McNeel. These devices are disadvantageous in that they are difficult to repair and require strict horizontal placement.
It is therefore an object of the present invention to provide a seismic sensor platform for retaining a plurality of seismic sensors in a configuration that results in a more accurate measurement of seismic reflection energy,
It is another object of the present invention to provide a seismic sensor platform that can be securely anchored in the earth at the bottom of tidal marine areas and fast moving rivers,
It is another object of the present invention to provide a seismic sensor platform that affords physical protection for the seismic sensors retained thereby and that also screens them from undesirable noise resulting from water and wind turbulence.
It i s another object of the present invention to provide a seismic sensor platform for retaining a plurality of seismic sensors in positions that permit them to be individually anchored in the earth.
It is another object of the present invention to provide a tension filter that is simple and effective in isolating strong surface currents from an anchored seismic sensor platform,
It is another object of the present invention to provide a seismic sensor platform for retaining a plurality of seismic sensors in groups to obtain more effective sampling of multiple vectors of reflected seismic energy.
It is another object of the present invention to provide a seismic sensor platform for retaining a plurality of unidirectional seismic sensors in positions that result in multidimensional sampling of reflected seismic energy.
It is another object of the present invention to provide a seismic sensor platform for retaining a plurality of seismic sensors in positions such that they may be easily replaced for repair or other purposes.
It is another object of the present invention to provide an elongated seismic platform for retaining at least three groups of rotatable seismic motion sensors,
It is another object of the present invention to provide a seismic sensing method in which seismic sensors of different natural frequency are employed to optimize multi frequency output.